Highlights from Recent Cancer Literature
Journal: Cancer Research (2012)

Driessens G, Beck B, Caauwe A, Simons BD, Blanpain C. Defining the mode of tumour growth by clonal analysis. Nature 2012;488:527–30.The presence of subpopulations of highly clonogenic cells, so-called cancer stem cells (CSC), within mouse and human tumors is usually analyzed by their potential to form tumors in immunodeficient mice. The existence of CSCs has never been proved in unperturbed tumors—until now. In a recent letter to the editor in Nature, Dreissens and colleagues describe their investigation of the fate of individual malignant cells and their search for CSCs in a natural microenvironment using the classical mouse cancer model of 7,12-dimethylbenz[α]anthracene/12-O-tetradecanoylphorbol-13-acetate (DMBA/TPA) skin carcinogenesis. In this model, mice were given a topical dose of a carcinogen DMBA (initiation), and then mitogenic TPA was repeatedly administered (promotion). After several weeks, slowly growing papillomas appeared on the skin of the mice and a fraction of these benign lesions progressed to invasive cancers. Using an inducible genetic lineage tracing system, Dreissen and colleagues were able to follow the fate of individual labeled cells within the papillomas over time. The authors pose 2 questions: Is the stem/progenitor cell hierarchy of normal tissue recapitulated in the tumors? And, are proliferative tumor cells equipotent or are they sustained by a distinct population of cells with stem-like properties? They found that the majority of tumor cells have only limited proliferative potential, whereas a fraction of cells persist for long periods, giving rise to progeny that occupy the bulk of the tumors. Papillomas are sustained by a cellular hierarchy in which a minority population of tumor cells with stem-like properties gives rise to a more transient progenitor cell pool. This mirrors the situation in the normal skin. The persistent cells in the papillomas have stem-cell–like characteristics and cycle twice a day, whereas the cells of the transient population cycle more slowly and terminally differentiate. The mode of tumor growth uncovered here also involves chance expansion and extinction of neighboring clones and may explain the clonal heterogeneity that has been found in many different tumors.Francia S, Michelini F, Saxena A, Tang D, de Hoon M, Anelli V, et al. Site-specific DICER and DROSHA RNA products control the DNA-damage response. Nature 2012;488:231–5.Maintaining genomic integrity is critical to preserve the viability of cells and organisms. Deleterious DNA lesions are common byproducts of cell metabolism and physiologic events such as meiosis and telomere shortening. DNA damage also occurs upon oncogenic activation and in response to various exogenous cues, including ionizing radiation and chemical carcinogens. An evolutionarily conserved DNA damage response (DDR) cascade detects DNA double-strand break (DSB) lesions and amplifies and transmits a signal to activate DNA repair machinery. DDR occurs rapidly and requires highly coordinated spatiotemporal dynamics involving specialized sensor, transducer, and effector proteins that assemble and disassemble at the site of DNA breaks. Although DDR is primarily controlled by posttranslational modifications of DDR proteins, the molecular events regulating the sequential recruitment of DDR proteins at DSB lesions remain poorly understood. Noncoding RNAs represent the vast majority of cellular RNAs and are found increasingly to be involved in such cellular processes as gene silencing, transcription, chromatin modification, mRNA splicing, and posttranslation as well as in spatial regulation of protein and protein complexes. Francia and colleagues identify a new class of small noncoding RNA processed by DICER and DROSHA RNAases, critical enzymes involved in microRNA biogenesis, that are necessary to activate DDR signaling in response to ionizing radiation and oncogene-induced genotoxic stress in human and mouse cells as well as in zebrafish embryos. The authors tested the effect of DICER and DROSHA inactivation on oncogene-induced senescence (OIS), one of the earliest cellular tumor-suppressive responses to aberrant oncogenic expression associated with DSB lesions and sustained DDR, leading to irreversible cell-cycle arrest. Strikingly, DICER and DROSHA inactivation impaired DDR foci formation following oncogenic RAS expression and prevented OIS-dependent cell-cycle arrest in primary human cells. Similar defects were also observed following DNA damage induced by ionizing radiation in human cells and in vivo in DICER-depleted zebrafish embryos. Notably, deregulation of DDR depends on a specific fraction of small cellular DROSHA and DICER RNA products (DDRNA) that are not involved in microRNA-dependent gene silencing. Interestingly, experiments performed using engineered murine cells harboring an inducible DSB locus coupled with deep sequencing analysis of small cellular RNAs revealed that DDRNAs are significantly enriched following DNA damage and are derived from sequences surrounding the locus of the DSB. Synthetic DDRNAs are sufficient to restore site-specific DDR signaling in RNA-depleted cells.Together, these findings reveal that DDRNAs are crucial for assembly of highly sequence-specific DDR foci and may play an enforcement role at cellular checkpoints during OIS, a critical tumor-suppressive response to genotoxic stress. It is also interesting to note that DICER inactivation was associated with tumor development in mice and humans, suggesting that loss of DDRNAs may contribute to tumorigenesis. The importance of DDRNAs in DDR is also supported by findings in the filamentous fungus Neurospora crassa and, more recently, in Arabidopsis and Drosophila species, in which analogous small RNAs and mechanisms also maintain genomic integrity. Further studies are needed to better understand how DDRNAs are generated and how they function in mammalian cells. An enhanced understanding of the basic mechanisms underlying DDR will open the possibility of new therapeutic avenues for controlling genomic stability and restraining aberrant cell growth and proliferation.Cavel O, Shomron O, Shabtay A, Vital J, Trejo-Leider L, Weizman N, et al. Endoneurial macrophages induce perineural invasion of pancreatic cancer cells by secretion of GDNF and activation of RET tyrosine kinase receptor. Cancer Res; Published OnlineFirst September 12, 2012; doi:10.1158/0008-5472.CAN-12-0764.Perineural invasion of cancer cells (CPNI) occurs in the majority of patients with pancreatic, prostate, and head and neck cancers. This process involves spread of cancer along nerve sheaths and is associated with a more aggressive phenotype in various malignancies. Although perineural invasion is frequently associated with a poor outcome, the precise molecular mechanism underlying this process is poorly understood. In this study, Cavel and colleagues demonstrate that endoneurial macrophages (EMΦ), which reside around nerves invaded by cancer, represent the key cell type mediating this unique type of cancer spreading. In some patients with pancreatic adenocarcinoma (PDA), significantly greater numbers of EMΦ are evident in the tumor beds. With the use of in vitro models of CPNI, it was shown that the EMΦs were recruited to the PDA tumor bed, and this migration of the EMΦ was induced by colony–stimulating factor-1. Further studies showed that EMΦs specifically induced a significant increase in the migratory ability of the PDA cells. Remarkably, the increased migratory ability of the PDA cells was due to glial-derived neurotrophic factor secretion by the EMΦ, which in turn activated RET tyrosine kinase receptor signaling pathways in the PDA cells. Furthermore, activity of the perineural invasion was markedly reduced in CCR2-deficient mice, which have a reduced recruitment and activation of macrophages, confirming a definitive role of EMΦ in perineural invasion and its cross-talk with the PDA cell. This study uncovers a novel approach for targeting perineural invasion and associated molecular pathways for cancer therapy.Sveen A, Agesen TH, Nesbakken A, Meling GI, Rognum TO, Liestol K, et al. ColoGuidePro: A prognostic 7-gene expression signature for stage III colorectal cancer patients. Clin Cancer Res; Published OnlineFirst September 18, 2012; doi:10.1158/1078-0432.CCR-11-3302.Patients with stage III colorectal cancer routinely receive adjuvant treatment with variable benefit. Molecular markers for prognostic stratification could help identify those patients with aggressive tumors who might respond to treatment. In this study, Sveen and colleagues analyzed gene expression signatures from stage II and III colorectal cancer samples from patients who did not receive adjuvant therapy to get a better sense of the natural history of the disease. They began with gene expression profiles from 95 stage II and III colorectal cancer samples. Using this as a “learning” series, 7 different signatures were then applied to a test set to select the best signature. A 7-gene signature was identified that included OLFM4, CXCL9, DMBT1, UGT2B17, SEMA3A, NT5E, and WNT11. Statistical methods were used to identify a signature that included genes that were not highly correlated with each other as a means to represent the breadth of biology correlated with aggressiveness in these tumors. The signature was then validated on an additional series. Samples were dichotomized into poor and favorable prognosis groups. Patients dichotomized into gene expression scores had 5-year relapse-free survival rates of 49% and 78%, respectively, in the test set and 45% and 81%, respectively, in the validation series. Importantly, the correlations between gene expression signature and outcome were seen using 2 different microarray platforms, suggesting robustness. In total, global gene expression profiles from 387 stage II and III colorectal cancer samples from 3 independent patient series were included in the study. The signature remained significant in multivariate models after adjustment for tumor stage and was independent of treatment. The authors argue that their results have clinical implications for patients whose general health may preclude chemotherapy as an option. Future studies will be required to compare this signature with other available signatures for colorectal cancer, such as Oncotype DX, to identify the best signatures for clinical implementation. Additional considerations include the requirement for frozen tissue followed by microarray analysis in this assay, as compared with the use of formalin-fixed paraffin-embedded tissue on assays such as Oncotype DX.Note: Breaking Advances are written by Cancer Research Editors. Readers are encouraged to consult the articles referred to in each item for full details on the findings described.